Sensor unit and musical instrument

ABSTRACT

A sensor unit capable of protecting a piezoelectric element and detecting vibration and sound is provided. The sensor unit comprises a sheet-like piezoelectric element having a porous layer, and a sound propagation sheet covering at least one face of the piezoelectric element and permitting transmission of sound from a first face toward a second face of the sound propagation sheet. A difference in acoustic pressure level between the sound incident on the sound propagation sheet and the transmitted sound is preferably no greater than 10 dB. A surface density of the sound propagation sheet is preferably from 0.03 g/m2 to 100 g/m2. The sound propagation sheet is preferably flexible and preferably has voids. The sensor unit preferably further comprises a sound insulation sheet covering another face of the piezoelectric element and substantially preventing transmission of sound from a second face toward a first face of the sound insulation sheet.

BACKGROUND OF THE INVENTION

The present invention relates to a sensor unit and a musical instrument.

DESCRIPTION OF THE RELATED ART

Conventionally, a vibration detection sensor has been known which ismounted on a vibrating part of a musical instrument and capable ofdetecting vibration of the vibrating part and outputting the vibrationas an electrical signal. As such a vibration detection sensor, a sensorhas been known in which a piezoelectric element is used, thepiezoelectric element comprising a porous resin film and electrodelayers disposed on both faces of the porous resin film (for example, seeJapanese Unexamined Patent Application, Publication No. 2010-89495).Such a sensor, in which the piezoelectric element with a porous layer isused, is suited for detection of sound owing to softness in a thicknessdirection, and does not inhibit vibration of the musical instrumentowing to lightweight properties and thinness. Therefore, such a sensorin which the piezoelectric element comprising a porous layer is used issuitably used as a pickup for a musical instrument that detects bothvibration and sound. It is to be noted that the term “sound” as referredto means a compressional wave transmitted through air, and the term“vibration” as referred to means vibration that propagates in a solid toa sensor.

In the case of using the aforementioned sensor in a musical instrument,etc., prevention of damage to the piezoelectric element is required inorder to maintain detection accuracy of the sensor. However, aprotection film covering the sensor for preventing damage to thepiezoelectric element may inhibit detection of sound.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2010-89495

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention was made in view of the foregoing circumstances,and an object of the present invention is to provide a sensor unit thatis capable of protecting a piezoelectric element and detecting bothvibration and sound, and a musical instrument comprising the sensorunit.

Means for Solving the Problems

According to an aspect of the invention made for solving theaforementioned problems, a sensor unit comprises: a sheet-likepiezoelectric element having a porous layer, wherein the sensor unitfurther comprises a sound propagation sheet that covers at least oneface of the piezoelectric element, and permits sound that is incident ona first face of the sound propagation sheet to be transmitted toward asecond face of the sound propagation sheet.

By virtue of the sound propagation sheet covering the one face of thepiezoelectric element, the sensor unit is enabled to protect from damagethe one face of the piezoelectric element that detects sound, andconsequently capable of maintaining sound detection accuracy. Inaddition, since the sound propagation sheet covering the one face of thepiezoelectric element permits the sound that is incident on the firstface of the sound propagation sheet toward the second face of the soundpropagation sheet, the sound from the one face side of the sensor unitis less likely to be muffled by the sound propagation sheet, and thesensor unit is capable of detecting both vibration and sound.

A difference in an acoustic pressure level between the sound incident onthe sound propagation sheet and the sound transmitted through the soundpropagation sheet is preferably no greater than 10 dB. Due to using thesound propagation sheet in which the difference in an acoustic pressurelevel between the sound incident on the sound propagation sheet and thetransmitted sound is no greater than the upper limit, reliableinhibition of muffling of the sound incident on the first face of thesound propagation sheet is enabled, and consequently the maintenance ofthe sound detection accuracy is further facilitated. As a result, thesensor unit may serve as a microphone.

A surface density of the sound propagation sheet is preferably no lessthan 0.03 g/m² and no greater than 100 g/m². Due to using the soundpropagation sheet having the surface density falling within theaforementioned range, the reliable protection of the one face of thepiezoelectric element and the reliable inhibition of muffling of thesound incident on the first face of the sound propagation sheet aresimultaneously attained, and consequently the maintenance of sounddetection accuracy is further facilitated.

The sound propagation sheet is preferably flexible. Due to theflexibility, the sound propagation sheet is capable of covering thepiezoelectric element without pressing it, and consequently durabilityof the piezoelectric element is improved. In addition, due to theflexibility of the sound propagation sheet, propagation of the vibrationcaused by the sound incident on the first face of the piezoelectricelement is facilitated, and consequently the maintenance of the sounddetection accuracy is further facilitated. It is to be noted that theterms “flexible” and “flexibility” as referred to mean that, forexample, when a test piece of 5 mm in width and 10 mm in length issupported on one shorter side thereof so as to be horizontally orientedat the support position, the distance between the positions of the twoopposed shorter sides in a vertical direction is no less than 5 mm.

The sound propagation sheet preferably has voids. Due to the soundpropagation sheet having voids, the sound incident on the first face ofthe sound propagation sheet is transmitted through the voids, andconsequently the propagation of the sound to the piezoelectric elementand the detection of the sound are further facilitated.

It is preferred that the sensor unit further comprises a soundinsulation sheet that covers another face of the piezoelectric elementand substantially prevents sound that is incident on a second face ofthe sound insulation sheet from being transmitted toward a first face ofthe sound insulation sheet. Due to covering the another face of thepiezoelectric element with the sound insulation sheet that substantiallyprevents sound incident on the second face of the sound insulation sheetfrom being transmitted toward the first face of the sound insulationsheet, the sound from the other face side of the sensor unit isprevented, and consequently the sound from the one face side of thesensor unit is more accurately detected. It is to be noted that theexpression “to substantially prevent sound from being transmitted” asreferred to means not only complete blockage of sound transmission butalso muffling of sound to such a degree that the sound escapes detectionby the piezoelectric element.

According to another aspect of the invention made for solving theaforementioned problems, a musical instrument comprises the sensor unitaccording to the aforementioned aspect.

The musical instrument is capable of detecting both the vibration andthe sound by virtue of the sensor unit, and consequently capable oftransforming an original tone of the musical instrument into anelectrical signal and outputting the electrical signal.

Effects of the Invention

As explained in the foregoing, the sensor unit and the musicalinstrument according to the aspects of the present invention are capableof detecting both the vibration and the sound while protecting thepiezoelectric element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a sensor unit according toa first embodiment of the present invention;

FIG. 2 is a schematic cross sectional view of a piezoelectric element ofFIG. 1;

FIG. 3 is a schematic cross sectional view of a sensor unit according toa second embodiment of the present invention;

FIG. 4 is a schematic cross sectional view of a sensor unit different inconstitution from the sensor unit of FIG. 3;

FIG. 5 is a schematic cross sectional view of a sensor unit according toa third embodiment of the present invention;

FIG. 6 is a schematic cross sectional view of a sensor unit according toa fourth embodiment of the present invention;

FIG. 7 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit of FIG. 1;

FIG. 8 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from that of FIG. 7;

FIG. 9 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 and 8;

FIG. 10 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 9;

FIG. 11 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 10;

FIG. 12 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 11;

FIG. 13 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 12;

FIG. 14 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 13;

FIG. 15 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 14;

FIG. 16 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 15;

FIG. 17 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 16;

FIG. 18 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 17;

FIG. 19 is a schematic cross sectional view for explaining a mountingconfiguration of the sensor unit different from those of FIGS. 7 to 18;

FIG. 20 is a graph conceptually showing a relationship between afrequency of sound and detection sensitivity of the sensor unit;

FIG. 21A is a schematic perspective view of a box-shaped piezoelectricelement;

FIG. 21B is a schematic plan view of a configuration of thepiezoelectric element of FIG. 21A prior to assembly;

FIG. 22 is a schematic side view of an exemplary assembly configurationof the piezoelectric element different from FIG. 21A;

FIG. 23 is a schematic side view of an exemplary assembly configurationof the piezoelectric element different from those of FIGS. 21A and 22;

FIG. 24 is a schematic perspective view of a string instrumentcomprising the sensor unit of FIG. 1;

FIG. 25 is a schematic plan view of an inner side of a soundboard of thestring instrument of FIG. 24; and

FIG. 26 is a schematic cross sectional view of a sensor unit accordingto another embodiment.

FIG. 27 is a schematic cross sectional view of a sensor unit accordingto another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described in detailhereinafter, with appropriate reference to the drawings.

First Embodiment

Sensor Unit

A sensor unit 1 of FIG. 1 comprises a sheet-like piezoelectric element 2having a porous layer. The sensor unit 1 further comprises: a firstsound propagation sheet 3 a that covers one face of the piezoelectricelement 2 and permits sound that is incident on the first face thereofto be transmitted toward the second face thereof; and a second soundpropagation sheet 3 b that covers the another face of the piezoelectricelement 2 and permits sound that is incident on the second face thereoftoward the first face thereof.

<Piezoelectric Element>

The piezoelectric element 2 is formed in a plate-like shape,substantially rectangular in a planar view. The piezoelectric element 2comprises a porous layer 4 and a pair of electrode layers (firstelectrode layer 5 a and second electrode layer 5 b), as illustrated inFIG. 2. The piezoelectric element 2 generates voltage in accordance withan amount of compression of the porous layer 4.

(Porous Layer)

The principal component of the porous layer 4 is preferably anelectrically chargeable material. For example, by polypropylene (PP),polyethylene (PE), polyethylene terephthalate (PET), polyvinyl chloride,a polyolefin resin, a fluorine-containing resin, and the like. The term“principal component” as referred to means a component which is of thehighest content, for example a component of which content is 50% or moreby mass.

The porous layer 4 is typically formed by subjecting to a polarizationprocess a plate-like member comprising any of the aforementioned resinsas a principal component. A procedure for the polarization process is,for example, a procedure of injecting charge by applying a high voltageof direct current or pulsed current; a procedure of injecting charge byirradiation with an ionizing radiation such as γ-rays, electron beams,etc.; a procedure of injecting charge by corona discharge; and the like.

The lower limit of an average thickness of the porous layer 4 ispreferably 30 μm and more preferably 50 μm. Meanwhile, the upper limitof the average thickness of the porous layer 4 is preferably 150 μm andmore preferably 100 μm. When the average thickness of the porous layer 4is less than the lower limit, processibility may be impaired due toreduced strength. To the contrary, when the average thickness of theporous layer 4 is greater than the upper limit, efficiency of thepolarization process may be decreased.

The lower limit of a modulus of elasticity of the porous layer 4 in adirection vertical to the thickness direction is preferably 1 GPa andmore preferably 1.5 GPa. Meanwhile, the upper limit of the modulus ofelasticity of the porous layer 4 in the direction vertical to thethickness direction is preferably 3 GPa and more preferably 2.5 GPa.When the modulus of elasticity of the porous layer 4 in the directionvertical to the thickness direction is less than the lower limit, astrain in the direction vertical to the thickness direction may becomegreater and consequently the vibration detection accuracy may bedecreased. To the contrary, when the modulus of elasticity of the porouslayer 4 in the direction vertical to the thickness direction is greaterthan the upper limit, the porous layer 4 is less likely to follow theexpansion and contraction of the first electrode layer 5 a and thesecond electrode layer 5 b, and consequently the first electrode layer 5a and the second electrode layer 5 b may be likely to be separated fromthe porous layer 4. It is to be noted that the term “modulus ofelasticity” as referred to means a value measured pursuant to JIS-K7161(2014).

The lower limit of a modulus of elasticity of the porous layer 4 in thethickness direction is preferably 0.1 GPa and more preferably 0.3 GPa.Meanwhile, the upper limit of the modulus of elasticity of the porouslayer 4 in the thickness direction is preferably 10 GPa and morepreferably 2 GPa. When the modulus of elasticity of the porous layer 4in the thickness direction is less than the lower limit, a large errorin vibration detection may arise. To the contrary, when the modulus ofelasticity of the porous layer 4 in the thickness direction is greaterthan the upper limit, detection of slight vibration may be difficult.

The lower limit of a density of the porous layer 4 is preferably 0.2g/cm³ and more preferably 0.4 g/cm³. Meanwhile, the upper limit of thedensity of the porous layer 4 is preferably 0.8 g/cm³ and morepreferably 0.6 g/cm³. When the density of the porous layer 4 is lessthan the lower limit, the porous layer 4 may decrease in strength. Tothe contrary, when the density of the porous layer 4 is greater than theupper limit, deformation of the porous layer 4 may be insufficient andconsequently the vibration detection accuracy may be decreased.

The porous layer 4 has a plurality of pores 6. The shape and size of thepores 6 are not particularly limited; however, the lower limit of anaverage height of the pores 6 is preferably 1 μm, and more preferably 3μm, for example. Meanwhile, the upper limit of the average height of thepores 6 is preferably 30 μm and more preferably 15 μm. When the averageheight of the pores 6 is less than the lower limit, the deformation ofthe porous layer 4 may be insufficient. To the contrary, when theaverage height of the pores 6 is greater than the upper limit, theporous layer 4 may decrease in strength. It is to be noted that theaverage height of the pores 6 is obtained by measuring maximum lengthsof arbitrary 20 pores in the thickness direction on an arbitrary crosssection of the porous layer 4 in the thickness direction and bycalculating an arithmetic average of the maximum lengths.

The lower limit of the porosity of the porous layer 4 is preferably 20%and more preferably 30%. Meanwhile, the upper limit of the porosity ofthe porous layer 4 is preferably 80% and more preferably 70%. When theporosity of the porous layer 4 is less than the lower limit, thedeformation of the porous layer 4 may be insufficient, and consequentlythe detection accuracy may be insufficient. To the contrary, when theporosity of the porous layer 4 is greater than the upper limit, theporous layer 4 may decrease in strength. It is to be noted that the term“porosity” as referred to means a proportion of the pores per unitvolume. The porosity ε (%) may be obtained by the equation (1) belowbased on a mass W (g), an apparent volume V (cm³) of the porous layer 4,and a true density ρ (g/cm³). The true density ρ may be obtained by theequation (2) below based on a volume V₀ (cm³) of the porous layer 4having been heated by heat pressing at 200° C. for 5 min with a load of1 kg/cm², and then cooled by cool pressing. Furthermore, the porosity εmay be given by the equation (3) below, which is obtained by pluggingthe equation (2) into the equation (1).ε=(1−W/ρV)×100  (1)ρ=W/V ₀  (2)ε=1−V ₀ /V  (3)(Electrode Layer)

The first electrode layer 5 a and the second electrode layer 5 b areoverlaid on the both faces of the porous layer 4, respectively. Thefirst electrode layer 5 a and the second electrode layer 5 b areconnected to respective lead wires (not shown in the Figure), which arein turn connected to an output terminal (not shown in the Figure).

A material for forming the first electrode layer 5 a and the secondelectrode layer 5 b is not particularly limited as long as the materialis electrically conductive, and is, for example, various types of metalssuch as aluminum and silver; alloys of these metals; carbon; and thelike.

An average thickness of each of the first electrode layer 5 a and thesecond electrode layer 5 b is not particularly limited, and may be, forexample, 0.1 μm or more and 30 μm or less. When the average thickness ofeach of the first electrode layer 5 a and the second electrode layer 5 bis less than the lower limit, damages such as rupture may occur in thefirst electrode layer 5 a and the second electrode layer 5 b. To thecontrary, when the average thickness of each of the first electrodelayer 5 a and the second electrode layer 5 b is greater than the upperlimit, the vibration may not be accurately detected.

A procedure for overlaying the first electrode layer 5 a and the secondelectrode layer 5 b on the porous layer 4 is not particularly limited,and is exemplified by: vapor deposition of aluminum; printing withconductive carbon ink; application and drying of a silver paste; and thelike.

The porous layer 4 has the pores in an inner part thereof, and istherefore soft and prone to be scratched. In addition, the electrodelayer 5 formed on the surface of the porous layer 4 is also soft andprone to be scratched. Therefore, the piezoelectric element 2constituted of these layers is required to be covered with a sheet inorder to prevent scratches. The piezoelectric element 2 is covered witha sound propagation sheet for permitting the piezoelectric element 2 todetect sound.

<Sound Propagation Sheet>

The first sound propagation sheet 3 a and the second sound propagationsheet 3 b are substantially rectangular sheets formed from a material ofthe same type and each having such a size that a range surrounded by anouter periphery of the piezoelectric element 2 is covered in a planarview. The first sound propagation sheet 3 a covers the one face of thepiezoelectric element 2, while the second sound propagation sheet 3 bcovers the another face of the piezoelectric element 2. The first soundpropagation sheet 3 a and the second sound propagation sheet 3 b arearranged such that outer peripheries thereof substantially correspond toeach other in a planar view, and are fixed to each other oncircumferential edges thereof. Accordingly, the piezoelectric element 2is surrounded by the first sound propagation sheet 3 a and the secondsound propagation sheet 3 b. It is to be noted that a procedure forfixing the first sound propagation sheet 3 a and the second soundpropagation sheet 3 b is not particularly limited, and may be, forexample: fixing by an adhesive or a tacky material; fixing by insertingpins, such as stapling; and fixing by sewing.

The sensor unit 1 is disposed such that the another face of the secondsound propagation sheet 3 b is in contact with a surface of a vibratingbody P such as a musical instrument, which is a target of vibrationdetection. Since the first sound propagation sheet 3 a permits soundthat is incident on the first face thereof to be transmitted toward thesecond face thereof, the sensor unit 1 arranged as described abovemainly detects sound being propagated in the first sound propagationsheet 3 a and detects the vibration of the vibrating body P beingpropagated in the second sound propagation sheet 3 b.

The upper limit of the difference in an acoustic pressure level betweenthe sound incident on the first sound propagation sheet 3 a and thetransmitted sound is preferably 10 dB and more preferably 5 dB.Meanwhile, the lower limit of the difference in the acoustic pressurelevel is preferably 1 dB and more preferably 2 dB. When the differencein the acoustic pressure level is greater than the upper limit, theacoustic pressure level of the sound being propagated to thepiezoelectric element 2 may be too low, and the sound may be less likelyto be detected by the piezoelectric element 2. To the contrary, when thedifference in the acoustic pressure level is less than the lower limit,it may be difficult to maintain an effect of protecting thepiezoelectric element 2 provided by the first sound propagation sheet 3a. The difference in an acoustic pressure level between the soundincident on the first sound propagation sheet 3 a and the transmittedsound may be obtained in a relative manner, on the basis of a differencebetween results of detection of a test tone by, for example: thepiezoelectric element 2 of the sensor unit 1 in which the piezoelectricelement 2 is covered by the first sound propagation sheet 3 a; and thepiezoelectric element 2 of the sensor unit 1 from which the first soundpropagation sheet 3 a has been removed. In other words, the differencein an acoustic pressure level may be obtained in a relative manner,through comparison between: a signal level of the transmitted sounddetected in the state in which the piezoelectric element 2 is covered bythe first sound propagation sheet 3 a; and a signal level of theincident sound detected in the state in which the first soundpropagation sheet 3 a has been removed from the sensor unit 1.Specifically, the difference in an acoustic pressure level is measuredby, for example, arranging the aforementioned two types of sensor unitsand a speaker in an anechoic chamber, and emitting sound from thespeaker. In this case, a face, which is opposite to the measurementtarget, of each of the two types of sensor units is preferably shieldedwith a rigid body or a sound-absorbing member. The measurement forobtaining the difference in an acoustic pressure level is carried out,for example, with a frequency of no less than 100 Hz and no greater than5,000 Hz.

The lower limit of a surface density of the first sound propagationsheet 3 a and the second sound propagation sheet 3 b is preferably 0.03g/m² and more preferably 1 g/m². Meanwhile, the upper limit of thesurface density of the first sound propagation sheet 3 a and the secondsound propagation sheet 3 b is preferably 100 g/m² and more preferably50 g/m². When the surface density is less than the lower limit, thefirst sound propagation sheet 3 a and the second sound propagation sheet3 b may decrease in strength and consequently an effect of protectingthe piezoelectric element 2 provided by the first sound propagationsheet 3 a and the second sound propagation sheet 3 b may beinsufficient. To the contrary, when the surface density is greater thanthe upper limit, transmission of sound may be hindered, and consequentlyit may be difficult for the piezoelectric element 2 to detect the sound.

The first sound propagation sheet 3 a and the second sound propagationsheet 3 b are only required to be capable of permitting sound that isincident on the first/second face to be transmitted toward thesecond/first face, and a material for forming these sheets is notparticularly limited. For example, resins, metals, inorganic materials,organic materials, and the like may be used as the material for formingthe first sound propagation sheet 3 a and the second sound propagationsheet 3 b.

In the case of using a resin as the material for forming the first soundpropagation sheet 3 a and the second sound propagation sheet 3 b, aprincipal component of the material is exemplified by PET, PP,polystyrene (PS), polycarbonate (PC), polyphenylene sulfide (PPS),polymethyl methacrylate (PMMA), polyetherimide (PEI), polyimide (PI),polyethylene naphthalate (PEN), triacetyl cellulose (TAC), cyclicolefin-derived resins, and the like. Alternatively, a metal film ofaluminum, nickel, platinum, or the like may also be used as the firstsound propagation sheet 3 a or the second sound propagation sheet 3 b.Although the metal film needs to be thin in order to propagate sound,such a thin film may be easily broken. Given this, it is preferred thatthe metal film is formed so as to adhere to the surface of thepiezoelectric element 2, for example by vapor deposition. In this case,the metal film having a thickness of about 10 nm is capable ofpropagating sound. When a reduction in sound detection efficiency isacceptable, the metal film may have a greater thickness.

The first sound propagation sheet 3 a preferably has voids. Due to thesound propagation sheet 3 a having voids in an inner part, the soundthat is incident on the first face thereof is transmitted through thevoids toward the second face thereof, and consequently the propagationof the sound to the piezoelectric element 2 and the detection of thesound by the piezoelectric element 2 are further facilitated. The voidsformed in the first sound propagation sheet 3 a may be continuousthrough the first sound propagation sheet 3 a in the thicknessdirection. Due to the voids formed in the first sound propagation sheet3 a being continuous through the first sound propagation sheet 3 a inthe thickness direction, propagation of the sound incident on the firstface toward the second face side is facilitated.

As the first sound propagation sheet 3 a having voids, for example, anonwoven fabric, a woven fabric, paper having voids, a porous sheet, andthe like may be used. As the porous sheet, for example, a sheet of thesame type as the porous layer 4 may be used.

The first sound propagation sheet 3 a and the second sound propagationsheet 3 b are preferably flexible. Due to the flexibility, the firstsound propagation sheet 3 a and the second sound propagation sheet 3 bare deformable along the shape and compressive deformation of thepiezoelectric element 2. As a result, the first sound propagation sheet3 a and the second sound propagation sheet 3 b are capable of coveringthe piezoelectric element 2 without pressing it, and consequentlydurability of the piezoelectric element 2 is improved. In addition, dueto the flexibility of the first sound propagation sheet 3 a, propagationof the vibration caused by the sound incident on the first face thereoftoward the piezoelectric element 2 is facilitated, and consequently animprovement of the sound detection accuracy of the piezoelectric element2 is facilitated.

Both faces of the piezoelectric element 2 may be either fixed to thesecond face of the first sound propagation sheet 3 a and the first faceof the second sound propagation sheet 3 b, respectively, or may not befixed thereto. In the case in which the piezoelectric element 2 is notfixed to the first sound propagation sheet 3 a and the second soundpropagation sheet 3 b, the piezoelectric element 2 would not bedistorted along the first sound propagation sheet 3 a and the secondsound propagation sheet 3 b, and more accurate detection of the soundand the vibration by the piezoelectric element 2 is facilitated. It isto be noted that, in the case of fixing the both faces of thepiezoelectric element 2 to the first sound propagation sheet 3 a or thesecond sound propagation sheet 3 b, a fixing procedure is notparticularly limited, and may be, for example: fixing by an adhesive ora tacky material; or fixing by friction between a face of thepiezoelectric element 2 and a face of the first sound propagation sheet3 a, or between a face of the piezoelectric element 2 and a face of thesecond sound propagation sheet 3 b.

In FIG. 1, the first sound propagation sheet 3 a and the second soundpropagation sheet 3 b are fixed to each other on circumferential edgesthereof in a planar view; however, the first sound propagation sheet 3 aand the second sound propagation sheet 3 b may be an integrated soundpropagation sheet. For example, the first sound propagation sheet 3 aand the second sound propagation sheet 3 b may be formed as a singlebag-like sound propagation sheet.

<Advantages>

By virtue of the first sound propagation sheet 3 a covering the one faceof the piezoelectric element 2, the sensor unit 1 is capable ofprotecting from damage the one face of the piezoelectric element 2 thatdetects sound, and consequently capable of maintaining the sounddetection accuracy. In addition, since the first sound propagation sheet3 a covering the one face of the piezoelectric element 2 permits thesound that is incident on the first face of the sound propagation sheet3 a to be transmitted toward the second face thereof, the sound enteringthe sensor unit 1 from the one face side thereof is less likely to bemuffled by the first sound propagation sheet 3 a, and the sensor unit 1is capable of detecting both vibration of the vibrating body P and thesound from the one face side. Therefore, the sensor unit 1 used as apickup for a musical instrument facilitate reproduction of an originaltone of the musical instrument.

In addition, since the second sound propagation sheet 3 b covers theanother face of the piezoelectric element 2, the sensor unit 1 iscapable of detecting sound entering the sensor unit 1 from the otherface side as well, and damage to the another face of the piezoelectricelement 2 is prevented while the sensor unit 1 is mounted to thevibrating body P.

Second Embodiment

In the sensor unit 11 of FIG. 3, a sound propagation sheet 13 isdisposed to cover the one face of the piezoelectric element 2. In thesensor unit 11, no sound propagation sheet is disposed between thepiezoelectric element 2 and the vibrating body P, and therefore theanother face of the piezoelectric element 2 is in direct contact withthe surface of the vibrating body P. It is to be noted that thepiezoelectric element 2 in the sensor unit 11 of FIG. 3 is identical tothe piezoelectric element 2 in the sensor unit 1 of FIG. 1, andexplanation thereof will be omitted through designating the identicalreference numeral.

<Sound Propagation Sheet>

As the sound propagation sheet 13, a sheet of the same type as the firstsound propagation sheet 3 a in the sensor unit 1 of FIG. 1 may be used.As illustrated in FIG. 3, the sound propagation sheet 13 is fixed to theone face of the piezoelectric element 2 so as to entirely cover the oneface of the piezoelectric element 2. As a result, prevention of damageto the piezoelectric element 2 is enabled. Due to the sound propagationsheet 13 permitting sound that is incident on the first face to betransmitted toward the second face, the piezoelectric element 2 iscapable of detecting the sound from the one face thereof. A procedurefor fixing the sound propagation sheet 13 to the piezoelectric element 2is not particularly limited, and the sound propagation sheet 13 is fixedto the one face of the piezoelectric element 2 by, for example, anadhesive or a tacky material.

Next, the sensor unit 12 according to another configuration of thepresent embodiment is illustrated in FIG. 4. A sound propagation sheet14 in the sensor unit 12 has such a size that a range surrounded by anouter periphery of the piezoelectric element 2 is covered in a planarview, and covers the one face of the piezoelectric element 2. Acircumferential edge of the sound propagation sheet 14 is fixed to thesurface of the vibrating body P. As described above, since the soundpropagation sheet 14 is fixed to the surface of the vibrating body P,the one face of the piezoelectric element 2 is not required to be fixedto the second face of the sound propagation sheet 14. Due to not fixingthe one face of the piezoelectric element 2 to the second face of thesound propagation sheet 14, the piezoelectric element 2 would not bedistorted with expansion and contraction of the sound propagation sheet14 and is therefore capable of detecting the sound with accuracy.

In the sensor unit 12 of FIG. 4, the another face of the piezoelectricelement 2 is not required to be fixed to the surface of the vibratingbody P. When the piezoelectric element 2 is not fixed to the vibratingbody P, the piezoelectric element 2 would not be distorted withexpansion and contraction of the vibrating body P and is thereforecapable of detecting the vibration of the vibrating body P withaccuracy.

<Advantages>

Due to the another face of the piezoelectric element 2 being in directcontact with the surface of the vibrating body P, the sensor unit 11 andthe sensor unit 12 are capable of detecting more accurately thevibration of the vibrating body P.

Third Embodiment

A sensor unit 21 of FIG. 5 comprises a sheet-like piezoelectric element2 having a porous layer. The sensor unit 21 further comprises: a firstsound propagation sheet 3 a that covers the one face of thepiezoelectric element 2 and permits sound that is incident on the firstface thereof to be transmitted toward the second face thereof; and asound insulation sheet 27 that covers the another face of thepiezoelectric element 2 and substantially prevents sound that isincident on the second face thereof from being transmitted toward thefirst face thereof. It is to be noted that the first sound propagationsheet 3 a and the piezoelectric element 2 in the sensor unit 21 of FIG.5 are identical to the sound propagation sheet 3 a and the piezoelectricelement 2 in the sensor unit 1 of FIG. 1, and explanation thereof willbe omitted through designating the identical reference numerals.

<Sound Insulation Sheet>

The sound insulation sheet 27 is a substantially rectangular sheethaving such a size that a range surrounded by an outer periphery of thepiezoelectric element 2 is covered in a planar view, and may be, forexample, a rigid body such as a metal plate. The sound insulation sheet27 is disposed such that the second face is fixed to the surface of thevibrating body P and the first face is in contact with the another faceof the piezoelectric element 2. In addition, the circumferential edge ofthe first sound propagation sheet 3 a covering the one face of thepiezoelectric element 2 is fixed to a circumferential edge of the firstface of the sound insulation sheet 27.

The sound insulation sheet 27 substantially prevents sound that isincident on the second face from being transmitted toward the firstface. As a result, sound propagated from the vibrating body P side islargely muffled, and sound from the one face side of the sensor unit 21,i.e., from the external space side, may be preferentially detected bythe piezoelectric element 2. Consequently, the piezoelectric element 2is capable of detecting more accurately the sound from the externalspace side.

The lower limit of the difference in an acoustic pressure level betweenthe sound incident on the sound insulation sheet 27 and the transmittedsound is preferably 50 dB and more preferably 60 dB. Meanwhile, theupper limit of the difference in the acoustic pressure level ispreferably 100 dB and more preferably 90 dB. When the difference in theacoustic pressure level is less than the lower limit, the sound from thevibrating body P side is more likely to be detected by the piezoelectricelement 2, and consequently the detection accuracy of the sound from theexternal space side may be decreased. To the contrary, when thedifference in the acoustic pressure level is greater than the upperlimit, the sound insulation sheet 27 is required to have an increasedthickness and consequently the sensor unit 21 may be unnecessarily largein size.

The lower limit of a surface density of the sound insulation sheet 27 ispreferably 500 g/m² and more preferably 600 g/m². Meanwhile, the upperlimit of the surface density of the sound insulation sheet 27 ispreferably 2,000 g/m² and more preferably 1,500 g/m². When the surfacedensity is less than the lower limit, the sound from the vibrating bodyP side may not be sufficiently muffled, and consequently the detectionaccuracy of the sound from the external space side may be decreased. Tothe contrary, when the surface density is greater than the upper limit,the sensor unit 21 may be excessively thick and unnecessarily large insize.

The sensor unit 21 of FIG. 5 may also be disposed upside down on thesurface of the vibrating body P. In other words, a face of the firstsound propagation sheet 3 a opposite to the piezoelectric element 2 maybe fixed to the surface of the vibrating body P, and the soundinsulation sheet 27 may face the external space side. When the soundinsulation sheet 27 is relatively large in mass, disposing the sensorunit 21 as described above facilitates propagation of the vibration fromthe vibrating body P to the piezoelectric element 2, since the soundinsulation sheet 27 also serves as a weight. Therefore, in the case ofpreferentially detecting the vibration from the vibrating body P, thesensor unit 21 disposed as described above is enabled to detect thevibration from the vibrating body P more accurately.

<Advantages>

Due to the sound insulation sheet 27 muffling the transmitted sound fromthe other face side of the sensor unit 21, the sensor unit 21 is capableof detecting more accurately the sound from the one face side.

Fourth Embodiment

A sensor unit 31 of FIG. 6 comprises a sheet-like piezoelectric element2 having a porous layer. The sensor unit 31 further comprises a soundpropagation sheet 33 that covers both faces of the piezoelectric element2 and permits sound that is incident on an outer face to be transmittedtoward a face on the piezoelectric element 2 side. It is to be notedthat the piezoelectric element 2 in the sensor unit 31 of FIG. 6 isidentical to the piezoelectric element 2 in the sensor unit 1 of FIG. 1,and explanation thereof will be omitted through designating theidentical reference numeral.

<Sound Propagation Sheet>

The sound propagation sheet 33 is, for example, a substantiallyrectangular sheet having a size at least twice as large as a plane areaof the piezoelectric element 2 in a planar view. The sound propagationsheet 33 is folded in half and disposed such that both faces of thepiezoelectric element 2 are entirely in contact with a face directedinward. As a result, both faces of the piezoelectric element 2 arecovered by the sound propagation sheet 33. The piezoelectric element 2of which both faces are covered by the sound propagation sheet 33 isdisposed such that one edge thereof is in contact with the surface ofthe vibrating body P. Both ends of the sound propagation sheet 33 thusfolded are further folded outward from the piezoelectric element 2, andfixed to the surface of the vibrating body P. Due to both ends of thesound propagation sheet 33 being fixed to the surface of the vibratingbody P, the sensor unit 31 is fixed to the vibrating body P. The sensorunit 31 is fixed to the vibrating body P such that the thicknessdirection of the piezoelectric element 2 is substantially parallel tothe surface of the vibrating body P. As the sound propagation sheet 33,a sheet of the same type as the first sound propagation sheet 3 a in thesensor unit 1 of FIG. 1 may be used.

Since the sensor unit 31 is disposed such that the thickness directionof the piezoelectric element 2 is substantially parallel to the surfaceof the vibrating body P, both faces of the piezoelectric element 2 facethe external space through the sound propagation sheet 33. Therefore,the sound from the external space is transmitted through the soundpropagation sheet 33 and detected by the piezoelectric element 2 at bothfaces thereof. Due to the piezoelectric element 2 being capable ofdetecting the sound from the external space at the both faces thereof,the sensor unit 31 is capable of detecting the sound from the externalspace more accurately.

<Advantages>

Due to the piezoelectric element 2 being capable of detecting the soundfrom the external space accurately at both faces thereof, the sensorunit 31 may be suitably used as a sensor to be embedded in a microphoneand the like.

Mounting Configuration of Sensor Unit

Next, mounting configurations of the sensor unit to the vibrating body Pwill be described. It is to be noted that in FIGS. 7 to 19 illustratingthe mounting configurations of the sensor unit, a sensor unit having thesame constitution as that of FIG. 1, 3 or 5 may be used.

<Mounting Configuration 1>

In the configuration illustrated in FIG. 7, a vibration non-transmittingmember 48 and a vibration-transmitting member 49 are arranged on thesurface of the vibrating body P. The vibration non-transmitting member48 and the vibration-transmitting member 49 each have a substantiallycuboid shape, and arranged such that respective lower faces are incontact with the surface of the vibrating body P and respective lateralfaces are in contact with each other. The sensor unit 1 is disposed suchthat the one face thereof faces the external space, and the other faceis in contact with respective upper faces of the vibrationnon-transmitting member 48 and the vibration-transmitting member 49. Thevibration non-transmitting member 48 and the vibration-transmittingmember 49 have substantially the same height (distance between the upperface and the lower face), and the upper face of the vibrationnon-transmitting member 48 and the upper face of thevibration-transmitting member 49 are substantially flush with eachother.

(Vibration Non-Transmitting Member)

The vibration non-transmitting member 48 is a member that is unlikely topropagate the vibration of the vibrating body P. Gel, a sponge, and thelike constituted of an organic material, an inorganic material, etc. maybe used as a material for forming the vibration non-transmitting member48.

(Vibration-Transmitting Member)

The vibration-transmitting member 49 is a member that is likely topropagate the vibration of the vibrating body P. Wood, ceramic, metal,and the like, for example, may be used as a material for forming thevibration-transmitting member 49. A rigid body formed from thesematerials, i.e., a matter formed by packing these materials withoutvoids, and the like, may be used as the vibration-transmitting member49. Alternatively, a material of the same type as the vibrating body Pmay be used as the vibration-transmitting member 49. Therefore, aprotruding part may be formed on the surface of the vibrating body P andused as the vibration-transmitting member.

In a region in which the other face side of the sensor unit 1 is incontact with the upper face of the vibration non-transmitting member 48,the vibration of the vibrating body P is unlikely to be propagated, andtherefore the piezoelectric element of the sensor unit 1 preferentiallydetects the sound from the external space. On the other hand, in aregion in which the other face of the sensor unit 1 is in contact withthe upper face of the vibration-transmitting member 49, the vibration ofthe vibrating body P is likely to be propagated, and therefore thepiezoelectric element preferentially detects the vibration of thevibrating body P. Therefore, a contact area between the sensor unit 1and the vibration non-transmitting member 48 as well as a contact areabetween the sensor unit 1 and the vibration-transmitting member 49 areadjusted through adjustments of the sizes of the vibrationnon-transmitting member 48 and the vibration-transmitting member 49 in aplanar view, or the like, whereby an adjustment of a ratio between thesound and the vibration to be detected by the piezoelectric element isenabled. As a result, a tone of an electronic musical instrument inwhich the sensor unit 1 is used as a pickup, for example, is enabled tobe controlled.

<Mounting Configuration 2>

In the configuration illustrated in FIG. 8, in addition to theconfiguration of FIG. 7, a sheet-like air vibration-insulating member 47is disposed on the one face of the sensor unit 1, in a regioncorresponding to the upper face of the vibration-transmitting member 49in a planar view. It is to be noted that, it is preferred that the airvibration-insulating member 47 is provided in an entire region coveringthe upper face of the vibration-transmitting member 49 in a planar view,but not in a region covering the upper face of the vibrationnon-transmitting member 48 in a planar view.

(Air Vibration-Insulating Member)

The air vibration-insulating member 47 is a member that is unlikely topropagate air vibration and likely to propagate vibration from a solid.In other words, due to the air vibration-insulating member 47 beingdisposed as in FIG. 8, propagation of sound from the external space tothe region on the one face of the sensor unit 1 in contact with the airvibration-insulating member 47 is inhibited. For example, a metal platemay be used as the air vibration-insulating member 47.

In the region corresponding to the upper face of thevibration-transmitting member 49 in a planar view in which the vibrationof the vibrating body P is preferentially detected, the airvibration-insulating member 47 more reliably inhibits detection of thesound from the external space, while the piezoelectric element iscapable of detecting the vibration of the vibrating body P moreaccurately by virtue of the air vibration-insulating member 47 servingas a weight.

<Mounting Configuration 3>

In the configuration illustrated in FIG. 9, instead of the sensor unit 1in the configuration of FIG. 7, a sensor unit 41 is provided. The sensorunit 41 has a sheet-like shape, and formed by, for example, providing avalley fold, a mountain fold, and a valley fold in this ordersubstantially in parallel to each other, in a part between one end andthe other end of the sensor unit 1. The sensor unit 41 is disposed suchthat, given a face on which the mountain fold protrudes being one face,the other face is in contact with the upper face of the vibrationnon-transmitting member 48 and the upper face of thevibration-transmitting member 49. In addition, the sensor unit 41 isdisposed such that a ridge line of the mountain fold corresponds to aboundary between the vibration non-transmitting member 48 and thevibration-transmitting member 49 in a planar view.

Due to the sensor unit 41 being disposed as described above, thevibration propagated by the vibration-transmitting member 49 can beprevented from being propagated to the vibration non-transmitting member48, and consequently more accurate detection of sound is enabled.

<Mounting Configuration 4>

In the configuration illustrated in FIG. 10, instead of the vibrationnon-transmitting member 48 in the configuration of FIG. 7, a vibrationnon-transmitting member 58 is provided having a substantially cuboidshape and greater in height than the vibration non-transmitting member48. In addition, instead of the sensor unit 1 in the configuration ofFIG. 7, a sensor unit 51 is provided having a shape to be in contactwith the upper face of the vibration-transmitting member 49 and theupper face of the vibration non-transmitting member 58. The sensor unit51 has a sheet-like shape, and formed by, for example, providing a foldon the sensor unit 1 such that the other face is in contact with theupper face of the vibration-transmitting member 49 and the upper face ofthe vibration non-transmitting member 58 arranged adjacent to eachother.

Due to the increased height of the vibration non-transmitting member 58,propagation of the vibration of the vibrating body P to a part of theother face of the sensor unit 51 in contact with the vibrationnon-transmitting member 58 is further inhibited. As a result, in aregion of the piezoelectric element corresponding to the upper face ofthe vibration non-transmitting member 58, sound detection accuracy isfurther improved.

<Mounting Configuration 5>

In the configuration illustrated in FIG. 11, instead of thevibration-transmitting member 49 in the configuration of FIG. 7, avibration-transmitting member 69 is provided having a substantiallytriangular prism shape. The vibration-transmitting member 69 has asubstantially right triangular lateral cross section, and is disposedsuch that two faces forming the right angle in the lateral cross sectionare in contact with the surface of the vibrating body P and a lateralface of the vibration non-transmitting member 48, respectively. Thelateral face of the vibration-transmitting member 69 in contact with thelateral face of the vibration non-transmitting member 48 hassubstantially the same height as the vibration non-transmitting member48.

In addition, instead of the sensor unit 1 of FIG. 7, a sensor unit 61 isprovided having a shape to be in contact with the upper face of thevibration non-transmitting member 48 and an inclined face of thevibration-transmitting member 69. The sensor unit 61 has a sheet-likeshape, and formed by, for example, providing a fold on the sensor unit 1such that the other face is in contact with the upper face of thevibration non-transmitting member 48 and the inclined face of thevibration-transmitting member 69 arranged adjacent to each other.

Due to using the vibration-transmitting member 69 having the faceinclined with respect to the surface of the vibrating body P, areduction in a distance between: a region for detecting the vibration onthe other face of the sensor unit 61; and the surface of the vibratingbody P is enabled. As a result, more accurate detection of the vibrationof the vibrating body P is enabled.

<Mounting Configuration 6>

In the configuration illustrated in FIG. 12, instead of the vibrationnon-transmitting member 48 in the configuration of FIG. 11, a vibrationnon-transmitting member 78 is provided having a substantiallyquadrangular prism shape with a substantially trapezoidal lateral crosssection. The lateral cross section of the vibration non-transmittingmember 78 is a trapezoid in which two internal angles are right anglesand bases are different in length. The vibration non-transmitting member78 is disposed such that a lower face, which corresponds to a sideforming the right angles with the bases in the lateral cross section, isin contact with the surface of the vibrating body P. Meanwhile, thevibration non-transmitting member 78 is disposed such that a lateralface, which includes a shorter base of the trapezoid as the lateralcross section, is in contact with a lateral face of thevibration-transmitting member 69. The lateral face of the vibrationnon-transmitting member 78 and the lateral face of thevibration-transmitting member 69 being in contact with each other havesubstantially the same height, and an inclination angle of an upper faceof the vibration non-transmitting member 78 is the same as that of theinclined face of the vibration-transmitting member 69. Therefore, theupper face of the vibration non-transmitting member 78 and the inclinedface of the vibration-transmitting member 69 are substantially flushwith each other.

In addition, instead of the sensor unit 61 of FIG. 11, a sensor unit 71is provided having a shape to be in contact with the upper face of thevibration non-transmitting member 78 and an inclined face of thevibration-transmitting member 69. The sensor unit 71 has a planar shapein which both faces are plane.

Due to using the vibration non-transmitting member 78 and thevibration-transmitting member 69 configured such that the upper face ofthe vibration non-transmitting member 78 and the inclined face of thevibration-transmitting member 69 are substantially flush with eachother, the other face of the planar sensor unit 71 is enabled to be incontact with both of the upper face of the vibration non-transmittingmember 78 and the inclined face of the vibration-transmitting member 69.As a result, accurate detection of the vibration of the vibrating bodyP, and easy formation of the sensor unit 71 without the need for bendingprocessing or the like of the sensor unit 71, are enabled.

<Mounting Configuration 7>

In the configuration illustrated in FIG. 13, the vibrationnon-transmitting member 48 and the vibration-transmitting member 49 inthe configuration of FIG. 7 are arranged at an interval. In other words,in the configuration illustrated in FIG. 13, a gap is provided betweenthe vibration non-transmitting member 48 and the vibration-transmittingmember 49. As a result, in the configuration illustrated in FIG. 13, thelower face of the sensor unit 1 has: a part in contact with the upperface of the vibration non-transmitting member 48; a part in contact withthe upper face of the vibration-transmitting member 49; and a partfacing the external space between these members. As a result, in aplanar view, the sensor unit 1 has: a region corresponding to the upperface of the vibration non-transmitting member 48; a region correspondingto the upper face of the vibration-transmitting member 49; and anunsupported region between these regions.

Due to arranging the vibration non-transmitting member 48 and thevibration-transmitting member 49 at an interval, avoidance ofinterference between the vibration non-transmitting member 48 and thevibration-transmitting member 49 is enabled. As a result, thepiezoelectric element in the sensor unit 1 achieves improvements in thesound detection accuracy in the region corresponding to the upper faceof the vibration non-transmitting member 48 in a planar view; and thedetection accuracy for the vibration of the vibrating body P in theregion corresponding to the upper face of the vibration-transmittingmember 49 in a planar view.

<Mounting Configuration 8>

In the configuration illustrated in FIG. 14, the vibrationnon-transmitting member 48 and the vibration-transmitting member 49 inthe configuration of FIG. 7 are arranged at an interval, and asound-absorbing member 50 having a substantially cuboid shape isdisposed between the vibration non-transmitting member 48 and thevibration-transmitting member 49. In the configuration illustrated inFIG. 14, one lateral face of the vibration non-transmitting member 48 isin contact with the other lateral face of the sound-absorbing member 50,and one lateral face of the sound-absorbing member 50 is in contact withthe other lateral face of the vibration-transmitting member 49. As aresult, in the configuration illustrated in FIG. 14, the lower face ofthe sensor unit 1 has: a part in contact with the upper face of thevibration non-transmitting member 48; a part in contact with the upperface of the sound-absorbing member 50; and a part in contact with theupper face of the vibration-transmitting member 49, which are continuousin one direction. As a result, in a planar view, the sensor unit 1 has:a region corresponding to the upper face of the vibrationnon-transmitting member 48; a region corresponding to the upper face ofthe sound-absorbing member 50; and a region corresponding to the upperface of the vibration-transmitting member 49, which are continuous inone direction. As a specific constitution of the sound-absorbing member50, various kinds of constitutions providing sound absorbency may beemployed. For example, a nonwoven fabric, a woven fabric, a memberproduced by covering a nonwoven fabric or a woven fabric with asynthetic resin, and the like may be used.

Due to the sound-absorbing member 50 disposed between the vibrationnon-transmitting member 48 and the vibration-transmitting member 49, areduction of interference between the vibration non-transmitting member48 and the vibration-transmitting member 49 is enabled. As a result, thepiezoelectric element in the sensor unit 1 achieves improvements in thesound detection accuracy in the region corresponding to the upper faceof the vibration non-transmitting member 48 in a planar view; and thedetection accuracy for the vibration of the vibrating body P in theregion corresponding to the upper face of the vibration-transmittingmember 49 in a planar view.

<Mounting Configuration 9>

In the configuration illustrated in FIG. 15, the vibrationnon-transmitting member 48 and the vibration-transmitting member 49 inthe configuration of FIG. 7 are arranged at an interval, and a buffer 60having a substantially cuboid shape is disposed between the vibrationnon-transmitting member 48 and the vibration-transmitting member 49. Inthe configuration illustrated in FIG. 15, one lateral face of thevibration non-transmitting member 48 is in contact with the otherlateral face of the buffer 60, and one lateral face of the buffer 60 isin contact with the other lateral face of the vibration-transmittingmember 49. As a result, in the configuration illustrated in FIG. 15, thelower face of the sensor unit 1 has: a part in contact with the upperface of the vibration non-transmitting member 48; a part in contact withthe upper face of the buffer 60; and a part in contact with the upperface of the vibration-transmitting member 49, which are continuous inone direction. As a result, in a planar view, the sensor unit 1 has: aregion corresponding to the upper face of the vibration non-transmittingmember 48; a region corresponding to the upper face of the buffer 60;and a region corresponding to the upper face of thevibration-transmitting member 49, which are continuous in one direction.As a specific constitution of the buffer 60, a constitution may beemployed that is: able to appropriately propagate the sound and thevibration, less likely to propagate the sound than the vibrationnon-transmitting member 48 is; and less likely to propagate thevibration than the vibration-transmitting member 49 is. For example, afoamed member having a plurality of pores generated by a foaming agentmay be used.

Due to being disposed between the vibration non-transmitting member 48and the vibration-transmitting member 49, the buffer 60 appropriatelypropagates the sound and the vibration. Thus, the piezoelectric elementin the sensor unit 1 is capable of detecting rich sound and vibration.In addition, the piezoelectric element is enabled to detect the soundand the vibration with a desired sensitivity, through adjustment ofphysical properties of the buffer 60 such as elasticity and density.

<Mounting Configuration 10>

In the configuration illustrated in FIG. 16, a sensor unit 81 a, avibration non-transmitting member 88 a, and a sensor unit 81 b arelaminated, in this order, on the surface of the vibrating body P. Thevibration non-transmitting member 88 a is a sheet-like member in whichboth faces are plane, having such a size that a range surrounded by anouter periphery of the sensor unit 81 a and the sensor unit 81 b iscovered in a planar view. The sensor unit 81 a and the sensor unit 81 beach have the same shape as, for example, the sensor unit 1 of FIG. 1.As the vibration non-transmitting member 88 a, a material of the sametype as, for example, the vibration non-transmitting member 48 of FIG. 7may be used.

The sensor unit 81 a disposed on the other face of the vibrationnon-transmitting member 88 a is in direct contact with the surface ofthe vibrating body P, and detects principally the vibration of thevibrating body P. On the other hand, the sensor unit 81 b disposed onthe one face of the vibration non-transmitting member 88 a is capable ofdetecting the sound from the external space accurately, since thevibration non-transmitting member 88 a inhibits propagation of thevibration of the vibrating body P. Therefore, through an adjustment of asurface area ratio between the sensor unit 81 a and the sensor unit 81b, an adjustment of a ratio between the sound and the vibration to bedetected is enabled.

<Mounting Configuration 11>

In the configuration illustrated in FIG. 17, the sensor unit 81 b in theconfiguration of FIG. 16 has been divided into a sensor unit 81 c and asensor unit 81 d, which are arranged on the one face of the vibrationnon-transmitting member 88 a.

Due to the divided sensor units for sound detection that enableselection of the sensor unit for sound detection, an adjustment of aratio between the sound and the vibration to be detected is facilitated.

Although the sensor unit for sound detection has been divided into twoin FIG. 17, the sensor unit for sound detection may also be divided intothree or more. In addition, the sensor unit 81 a for vibration detectionmay also be divided.

<Mounting Configuration 12>

In the configuration illustrated in FIG. 18, an air vibration-insulatingmember 87 is further disposed between the sensor unit 81 a and thevibration non-transmitting member 88 a in the configuration of FIG. 16.The air vibration-insulating member 87 is a sheet-like member in whichboth faces are plane, having such a size that a range surrounded by anouter periphery of the sensor unit 81 a is covered in a planar view, andthe air vibration-insulating member 87 may be, for example, a metalplate.

Due to the air vibration-insulating member 87 being thus disposed,inhibition of propagation of the sound through the vibrationnon-transmitting member 88 a to the sensor unit 81 a is enabled, andconsequently an improvement in the detection accuracy of the sensor unit81 a for the vibration of the vibrating body P is attained.

<Mounting Configuration 13>

In the configuration illustrated in FIG. 19, a vibrationnon-transmitting member 88 b and a sensor unit 81 e are further disposedin this order on the one face side of the sensor unit 81 b, in additionto the configuration of FIG. 16. The sensor unit 81 e has the same shapeas, for example, the sensor unit 81 a. In addition, the vibrationnon-transmitting member 88 b has the same shape as, for example, thevibration non-transmitting member 88 a.

Since the vibration non-transmitting member 88 a and the vibrationnon-transmitting member 88 b are unlikely to propagate the vibration butlikely to propagate the sound, the sensor unit 81 b is unlikely todetect the vibration from the vibrating body P but likely to detect thesound from the external space. Therefore, the vibration of the vibratingbody P is detected by the sensor unit 81 a, while the sound from theexternal space is detected by the sensor unit 81 b and the sensor unit81 e. In other words, the configuration of FIG. 19 enables an area ofthe sensor unit for sound detection to be greater than an area forvibration detection, and in turn enables an increase in a ratio of thesound in the detection.

Furthermore, a lamination including a greater number of vibrationnon-transmitting members and sensor units enables a further increase inthe ratio of the sound in the detection to be achieved.

In this case, the vibration non-transmitting member is preferably formedfrom a material that is likely to propagate sound, for example amaterial having continuous pores such as a sponge.

Configuration Example of Piezoelectric Element

Next, a configuration example of the piezoelectric element provided inthe sensor unit of the present embodiment will be described.

In a case of detecting sound by using a piezoelectric element comprisinga rectangular planar porous body, a wavelength smaller than a width ofthe piezoelectric element is canceled and therefore undetectable.Therefore, a large area piezoelectric element has a lower sensitivityfor a higher frequency. However, in an odd-mode such as a third mode,there are wavelengths which are not canceled even in a high frequency,as shown in FIG. 20.

By reducing an area for sound detection in the piezoelectric element,sensitivity can be made flat even in a higher frequency; however, insuch a case, sensitivity is reduced due to a reduced capacitance. Thereduction of capacitance may be inhibited through an increase in asurface area of the piezoelectric element. The following configurationsof the piezoelectric element enable the increase in the surface area,and in turn the highly sensitive detection of sound even in a highfrequency.

Configuration Example 1

A piezoelectric element 92 of FIG. 21A is formed in a box shape. Forexample, such a configuration in an open-top cuboidal box-like shapeenables an about fivefold increase in the surface area of thepiezoelectric element compared to that of a sheet-like configuration,leading to an inhibition of the reduction of capacitance.

The piezoelectric element 92 of FIG. 21A can be obtained by, forexample: forming a planar piezoelectric element in such a shape that asquare at the center has four squares connected thereto, each sharing aside with the square at the center in a planar view, as illustrated inFIG. 21B; and then bending the planar piezoelectric element thus formedalong the four sides of the square at the center.

Configuration Example 2

A piezoelectric element 102 of FIG. 22 is configured such that thepiezoelectric element 102 formed in a sheet-like shape is folded aroundouter peripheries of a plurality of cylindrical spacers 103.Specifically, the piezoelectric element 102 is formed by: arranging theplurality of spacers 103 substantially in parallel with each other, sothat the piezoelectric element to be folded around the outer peripheriesthereof would be substantially parallel across the spacers 103; and thenfolding the sheet-like piezoelectric element around the plurality ofspacers 103. For example, in the case of the piezoelectric element 102of FIG. 22, an about fivefold increase in the surface area is enabled,compared to an unfolded piezoelectric element having the same planearea.

Configuration Example 3

A piezoelectric element 112 of FIG. 23 is configured such that thepiezoelectric element 112 formed in a sheet-like shape is folded at aplurality of positions in an accordion shape, supported by cylindricalspacers 103. Specifically, the piezoelectric element 112 is formed by:arranging the plurality of spacers 103 substantially parallel, at suchpositions that each fold of the sheet-like piezoelectric element ismaintained by insertion between two adjacent spacers 103; and thenengaging the sheet-like piezoelectric element folded in an accordionshape with the plurality of spacers 103. For example, in the case of thepiezoelectric element 112 of FIG. 23, an about six-fold increase in thesurface area is enabled, compared to an unfolded piezoelectric elementhaving the same plane area.

It is to be noted that the piezoelectric elements shown in FIGS. 21A,22, and 23 may be installed in any desired orientation.

<String Instrument>

A string instrument 121 of FIGS. 24 and 25 comprises principally: ahollow body 123 having a soundboard 122; a bridge 125 that is providedon an outer face side of the soundboard 122 and supports a plurality ofstrings 124; a saddle 126 provided on an outer face of the bridge 125; aneck 127 that is coupled to the body 123 and extends from one end sideof the soundboard 122; and a head 128 provided on one end side of theneck 127. First end sides of the plurality of strings 124 are twistedaround and fastened to a plurality of pegs 129 provided on the head 128,respectively, while second end sides of the strings 124 are supported bythe bridge 125 through the saddle 126, and are fastened to the pluralityof pins 130, respectively. The soundboard 122 has a sound hole 131between a second end of the neck 127 and the bridge 125.

As illustrated in FIG. 25, a plurality of struts 132 are attached to aninner face of the soundboard 122. In addition, on the inner face of thesoundboard 122, a plate 133 positioned opposite to the bridge 125 acrossthe soundboard 122, and a reinforcement board 134 for reinforcing thesoundboard 122 are provided.

The string instrument 121 comprises the sensor unit 1 of FIG. 1. Thesensor unit 1 is mounted on an inner face of the plate 133. In otherwords, in the string instrument 121, the plate 133 serves as thevibrating body P, and the sensor unit 1 is provided on the surface ofthe vibrating body P. The string instrument 121 is configured as anacoustic-electric guitar that converts the vibration of the strings 124into an electrical signal by means of the sensor unit 121, and outputsthe electrical signal.

<Advantages>

In the string instrument 121, the sensor unit 1 is capable of detectingsound generated by the vibration of the vibrating body P and resonanceof the body 123 with the vibration of the plurality of strings 124, andconsequently an original tone of the musical instrument is convertedinto an electrical signal and the electrical signal is output.

Other Embodiments

The embodiments described above do not restrict the constituent featuresof the present invention. Therefore, any omission, substitution andaddition of each of the constituent features of the embodiments can bemade on the basis of the description of the present specification andcommon general technical knowledge, and such omitted, substituted and/oradded features are to be construed to entirely fall under the scope ofthe present invention.

For example, as illustrated in FIG. 26, the sensor unit 11 of the secondembodiment may also be disposed in an orientation different from that ofFIG. 3. In other words, the sensor unit 11 may also be disposed suchthat the sound propagation sheet 13 is in contact with the surface ofthe vibrating body P. In this case, for example by using a high strengthmaterial for an electrode layer on an opposite side to the soundpropagation sheet 13 of the piezoelectric element 2, accurate detectionof sound is enabled while damage to the porous layer is inhibited.

Since the sound propagation sheet disposed on an opposite side to thevibrating body of the sensor units of the first embodiment, the secondembodiment, and the third embodiment also serves as a weight, anadjustment of the thickness or a mass of the sound propagation sheetenables alteration of the characteristics to be detected by thepiezoelectric element. In addition, a weight 47 may be provided on anopposite side to the vibrating body P of the sensor unit 1 (FIG. 27).Specifically, a sheet formed from a synthetic resin, or a sheet or aplate formed from a metal may be provided on an opposite side to thevibrating body of the sensor unit. In the case in which such asheet-like weight is provided, the weight may have through-holes forfacilitating the propagation of the sound. Furthermore, a cushion layermay also be provided between the sensor unit and the vibrating body. Dueto the cushion layer being provided, a reduction of the vibrationtransmitted from the vibrating body to the sensor unit, and in turn anincrease in the ratio of the sound in the detection, are enabled.

The sensor unit is not necessarily mounted on an acoustic-electricguitar. The sensor unit may also be mounted on various types of stringinstruments such as a classic guitar, a violin, a cello, a mandolin, apiano, and the like, as well as musical instruments other than stringinstruments, such as percussion instruments. In other words, the musicalinstrument according to the present invention is not necessarily astring instrument, and may also be configured as a percussion instrumentor the like. In addition, a mounting position of the sensor unit is notparticularly limited, and the sensor unit may be mounted on an arbitraryvibrating body of a musical instrument. Furthermore, the sensor unit tobe mounted on the musical instrument is not limited to the sensor unitof FIG. 1, and any of the sensor units in the aforementioned embodimentmay be used.

The sensor unit may be configured as a pickup for a musical instrumentto be mounted on a musical instrument, but may also be used innon-musical instruments such as a boundary microphone.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, the sensor unit according to theembodiment of the present invention is capable of detecting both thevibration and the sound while protecting the piezoelectric element.Therefore, the sensor unit is suitably used not only in various types ofmusical instruments, but also in buildings, machinery, transportation,etc. for detection of abnormal noise and sound as signs of failure.

Explanation of the Reference Symbols

-   1, 11, 12, 21, 31, 41, 51, 61, 71, 81 a, 81 b, 81 c, 81 d, 81 e    Sensor unit-   2, 92, 102, 112 Piezoelectric element-   3 a Sound propagation sheet-   3 b Sound propagation sheet-   4 Porous layer-   5 a First electrode layer-   5 b Second electrode layer-   6 Air hole-   13, 14, 33 Sound propagation sheet-   27 Sound insulation sheet-   47, 87 Air vibration-insulating member-   48, 58, 78, 88 a, 88 b Vibration non-transmitting member-   49, 69 Vibration-transmitting member-   50 Sound-absorbing member-   60 Buffer-   103 Spacer-   121 String instrument-   122 Soundboard-   123 Body-   124 String-   125 Bridge-   126 Saddle-   127 Neck-   128 Head-   129 Peg-   130 Pin-   131 Sound hole-   132 Strut-   133 Plate-   134 Reinforcement board-   P Vibrating body

The invention claimed is:
 1. A sensor unit comprising: a sheet-likepiezoelectric element having a porous layer; and a sound propagationsheet that covers at least one face of the piezoelectric element, thesound propagation sheet being configured to permit sound that isincident on a first face of the sound propagation sheet to betransmitted toward a second face of the sound propagation sheet, whereina surface density of the sound propagation sheet is no less than 0.03g/m² and no greater than 100 g/m², wherein the sound propagation sheetis flexible, and wherein the sound propagation sheet has voids.
 2. Thesensor unit according to claim 1, wherein a difference in an acousticpressure level between the sound incident on the sound propagation sheetand the sound transmitted through the sound propagation sheet is nogreater than 10 dB.
 3. The sensor unit according to claim 1, furthercomprising a sound insulation sheet that covers another face of thepiezoelectric element and substantially prevents sound that is incidenton a second face of the sound insulation sheet from being transmittedtoward a first face of the sound insulation sheet.
 4. A musicalinstrument comprising the sensor unit according to claim
 1. 5. A sensorunit comprising: a sheet-like piezoelectric element having a porouslayer; a sound propagation sheet that covers at least one face of thepiezoelectric element, the sound propagation sheet being configured topermit sound that is incident on a first face of the sound propagationsheet to be transmitted toward a second face of the sound propagationsheet; a vibrating body provided on a second face of the piezoelectricelement; and a sheet-like weight provided on the piezoelectric elementsuch that the sheet-like weight and the vibrating body are disposed onopposite sides of the piezoelectric element, wherein a surface densityof the sound propagation sheet is no less than 0.03 g/m² and no greaterthan 100 g/m², wherein the sound propagation sheet is flexible, andwherein the sound propagation sheet has voids.
 6. A musical instrumentcomprising the sensor unit according to claim 5.